1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus incorporating the same, and more particularly, to a fixing device that fixes a toner image in place on a recording medium with heat and pressure, and an electrophotographic image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine incorporating several of those imaging functions, incorporating such a fixing device.
2. Description of the Background Art
In electrophotographic image forming apparatus, such as photocopiers, facsimile machines, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by attracting toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process is followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium by melting and settling the toner with heat and pressure.
Various types of fixing devices are known in the art, most of which employ a pair of generally or at least partially cylindrical looped belts or rollers, one being heated for fusing toner (“fuser member”) and the other being pressed against the heated one (“pressure member”), which together form a heated area of contact called a fixing nip through which a recording medium is passed to fix a toner image under heat and pressure.
FIG. 1 is an end-on, axial cutaway view schematically illustrating a conventional fixing device 200, which employs a pair of generally cylindrical fixing members, one being a heated fuser belt 204 and the other being a pressure roller 205 pressed against the fuser belt 204.
As shown in FIG. 1, in the fixing device 200, the fuser belt 204 is looped into a partially cylindrical configuration along an outer surface of a cylindrical fuser roller 203 which is formed of an elastic material such as rubber. The fuser belt 204 is rotatable around the fuser roller 203 as well as a heat roller 202 internally equipped with a heater 201 that heats the length of the belt 204 upon contacting the heat roller 202. The pressure roller 205 is pressed against the fuser roller 203 via the fuser belt 204 to form a fixing nip N therebetween.
During operation, a recording medium S bearing an unfixed, toner image thereon enters the fixing nip N. As the pressure roller 205 rotates, the incoming sheet S advances together with the fuser belt 213 along the surface of the fuser roller 203, so as to fix the toner image in place with heat from the fuser belt 204 and pressure from the pressure roller 205.
FIG. 2 is an end-on, axial cutaway view schematically illustrating another conventional fixing device 210, which employs a fuser belt 213 formed of thin heat-resistant film rotatably held around a stationary, ceramic heater 211, in place of the fuser belt 204 entrained around the rotatable rollers 202 and 203.
As shown in FIG. 2, the fixing device 210 has a pressure roller 212 that rotates in pressure contact with the stationary heater 211 through the rotatable fuser film 213 to form a fixing nip N therebetween. As in the case with the fixing device 200, upon entry of a recording sheet into the fixing nip N, the pressure roller 212 rotates to advance the incoming sheet together with the fuser film 213 along the surface of the stationary heater 211, so as to fix the toner image in place with heat from the heater 211 through the fuser film 213 and pressure from the pressure roller 212.
The configuration based on the combination of the heat-resistant film 213 and the stationary heater 211 is commonly employed in a high-speed, on-demand printer, which can promptly execute a print job upon startup without significant energy consumption. The use of the heat-resistant film 213, which exhibits a relatively low heat capacity, and therefore can be swiftly heated, eliminates the need for preparing and keeping the heater 211 in a sufficiently heated state for immediate processing of an incoming print job, resulting in shorter periods of wait time required to execute an initial print job upon startup, as well as smaller amounts of energy wasted during standby time.
Although generally successful for its intended purpose, the conventional fixing device 210 has several drawbacks. One drawback is its vulnerability to wear, where the heat-resistant film 213 has its inner surface repeatedly brought into frictional contact with the surface of the stationary ceramic heater 211. The frictionally contacting surfaces of the film 213 and the heater 211 readily chafe and abrade each other, which, after a long period of operation, results in increased frictional resistance at the heater/film interface, leading to disturbed rotation of the fuser belt 213, or increased torque required to drive the pressure roller 212. If not corrected, such defects can eventually cause failures, such as displacement of a printed image caused by a recording sheet slipping through the fixing nip, and damage to a gear train driving the fixing members due to increased stress during rotation.
Another drawback of the fixing device 210 is the difficulty in maintaining a uniform processing temperature throughout the fixing nip N. The problem arises where the fuser film 213, which is once locally heated at the fixing nip N by the heater 211, gradually loses heat as it travels downstream the fixing nip N, so as to cause a large discrepancy in temperature between immediately downstream from the fixing nip N (where the fuser belt is hottest) and immediately upstream from the fixing nip N (where the fuser belt is coldest). Such thermal instability adversely affects fusing performance of the fixing device 210, particularly in a high-speed application where the rotational fixing member tends to dissipate higher amounts of heat during rotation at a high processing speed.
The former drawback of the fixing device 210 has been addressed by another, improved conventional fixing device, which uses a lubricant, such as a low-friction sheet of fiberglass impregnated with polytetrafluoroethylene (PTFE), disposed between the contacting surfaces of a stationary pressure pad and a rotatable fixing belt. In this improved fixing device, the rotatable fixing belt is looped for rotation around the stationary pressure pad, while held in contact with an internally heated, rotatable fuser roller that has an elastically deformable outer surface. The pressure pad is spring-loaded to press against the fuser roller through the fixing belt, which establishes a relatively large fixing nip therebetween as the fuser roller elastically deforms under pressure.
According to this arrangement, the provision of the lubricant sheet prevents abrasion and chafing at the interface of the stationary and rotatable fixing members, as well as concomitant defects and failures of the fixing device. Moreover, the relatively large fixing nip translates into increased efficiency in heating a recording sheet by conduction from the fuser roller, which allows for designing a compact fixing device with reduced energy consumption.
However, the improved conventional method does not address the thermal instability caused by locally heating the fixing belt at the fixing nip, as is the case with the conventional fixing device 210. Further, both conventional methods share a common problem due to the use of a fixing roller that exhibits a relatively high heat capacity and therefore takes time to heat up to a desired processing temperature, leading to increased periods of time required during warm-up. Hence, although designed to provide an increased thermal efficiency through use of a heat-resistant fuser film or an elastically deformable fuser roller, the conventional methods fail to provide satisfactory fixing performance for high-speed, on-demand applications.
To cope with the problems of the conventional fixing devices, a further improved method has been proposed, which employs a generally cylindrical, stationary tubular belt holder of thermally conductive material that can heat the entire length of a fuser belt held around its circumference and subjected to heating, instead of a stationary heater or heated roller that locally heats the fuser belt or film solely at the fixing nip. FIG. 3 is an end-on, axial cutaway view schematically illustrating a fixing device 220 employing a thermal belt holder 224.
As shown in FIG. 3, the fixing device 220 includes a fuser belt 221 looped into a generally cylindrical configuration, and a generally cylindrical pressure roller 222, both extending in an axial, longitudinal direction in which FIG. 3 is drawn. Disposed inside the loop of fuser belt 221 is a stationary, generally rigid fuser pad 223 against which the pressure roller 222 is pressed through the fuser belt 221 to form a fixing nip N therebetween.
The tubular belt holder 224 extends in the axial direction inside the loop of fuser belt 221, defining a longitudinal side slot 224a on one side thereof to accommodate the fuser pad 223 with a thermal insulator 225 disposed between the adjoining walls of the belt holder 224 and the fuser pad 223. The belt holder 224 is equipped with a reinforcing member 226 and a radiant heater 227 both extending in the axial direction inside the tubular belt holder.
During operation, the pressure roller 222 rotates clockwise in the drawing in contact with the fuser belt 221, which in turn rotates counterclockwise in the drawing around the stationary belt holder 224. The rotating belt 221 is guided by the circumference of the belt holder 224, which is internally heated with the radiant heater 227 to radially heat the fuser belt 221 during rotation.
In the fixing device 220, the thermal belt holder 224 is formed by bending a thin sheet of metal into a tubular configuration. The thin-walled belt holder 224 can swiftly conduct heat to the fuser belt 221, while guiding substantially the entire length of the belt 221 along the outer circumference thereof. Using the thin-walled conductive belt holder 224 thus allows for heating the fuser belt 221 swiftly and uniformly, resulting in shorter periods of warm-up time which meet high-speed, on-demand applications.
One difficulty encountered when using a thin-walled thermal belt holder is that the thin-walled tubular member is vulnerable to bending when subjected to high pressure applied perpendicular to the axial direction. Such deformation or displacement of the belt holder would result in inconsistencies in the width or strength of the fixing nip, adversely affecting the performance of the fixing device. To address this difficulty, the fixing device 220 employs the fuser pad 223 and the reinforcing member 226 to retain the belt holder 224 in shape and position to maintain a proper width and strength of the fixing nip N.
Specifically, the fixing device 220 forms the fixing nip N by pressing the pressure roller 222 against the fuser pad 223, which isolates the belt holder 221 from pressure at the fixing nip N. Formed of a rigid material, the fuser pad 223 is held stationary in both axial and transaxial directions to resist pressure from the pressure roller 222, thereby obtaining a desired width and strength of the fixing nip N. Using the stationary fuser pad 223 allows for use of an extremely thin-walled belt holder to maximize thermal efficiency in heating the fuser belt, leading to reduced warm-up time and reduced energy consumption of the fixing device.
In addition to the fuser pad 223 isolating the heat pipe from nip pressure, the fixing device 220 has the reinforcing member 226 to reinforce the belt holder 224 around the fixing nip N. The reinforcing member 226 comprises a T-beam having an axial cross-section in the shape of letter “T” with a flange (i.e., the vertical leg of the T) extending in a load direction in which the pressure roller 222 exerts nip pressure, held against the inner circumference of the belt holder 224 forming the wall of the side slot 224a. 
Although provided to resist pressure transmitted from the pressure roller 222 through the fuser pad 223, the T-beam reinforcement 226 often fails to withstand nip pressure, with its flange bending or deformed in the axial direction. Such deformation of the reinforcing member 226 translates into displacement or deformation of its surrounding structure, in particular, that of the fuser pad 223, resulting in variations in width and strength of the fixing nip N.
To provide the reinforcing member 226 with sufficient strength, a common approach is to form the T-beam with increased length, depth, and thickness. Unfortunately, such an approach is impractical as the reinforcing member 226 is required to be accommodated within a limited space inside the belt holder 224, which imposes limitations on the dimensions of the T-beam reinforcement 226. Moreover, positioning the larger reinforcing member 226 within the heat pipe can reduce efficiency in heating the fuser belt 221 with the belt holder 224, where the enlarged dimensions of the T-beam 226 prevent radiation from reaching the circumference of the belt holder 224, or where the larger thickness of the T-beam 226, exhibiting an increased heat capacity, tends to absorb a greater amount of heat.